A frequent application for insert-type fastening devices generally known as "Thread-Inserts" is the utilization of holes in the corner or side fillets or projecting mounting studs of molded or die-cast boxes or containers to which a lid or panel or a component part is to be affixed. This is the most common way of mounting and assembling appliances and instruments. The use of such thread inserts is very widespread in industrial production. Therefore, cost of the inserts and economy of insertion are of considerable importance. For example, the obvious methods of drilling and tapping the fillets or insert-molding thread bushings into the case, are by far too slow and expensive. They are hardly used nowadays in production.
According to the state of the art now known, thread inserts are commonly small metal bushings with an inside thread and a rough outside; generally, the bushings are knurled or grooved, or have sharp undercuts. They are frequently slotted in axial directions, so that the inserted screw may produce a slight expansion, or they may rely for anchoring on a springy circumferential component which has to be compressed for insertion and bears frictionally against the hole wall. It is characteristic of most known types of thread inserts that they must be force-fitted into the holes in the container or mounting base. This force-fitting characteristic applied also to those types of known thread inserts which have on their outside a coarse self-cutting thread so that they can be forcibly screwed into the holes in the mounting base. As a result, it occurs not infrequently that the force of the insertion cracks the plastic or die-cast mounting base, or sets up tensions and stresses which later result in cracks. If the insertion is less forcible, the insert may remain loose or may be subsequently loosened by vibration or may turn when a screw is inserted and tightened. The different expansion coefficients of the metal bushing and the surrounding plastic or die-cast material may also lead to loosening at a later date, especially under the influence of vibration.
In order to overcome the problems caused by forcible insertion of thread inserts, ultrasonic or spot-heating devices are used to insert the bushing into the hole without the use of force, by locally plasticizing or re-heating the material surrounding the bushing. Spot-heating devices are restricted to use on thermoplastic materials. They are comparatively slow to operate, and they are not always fully reliable in use if there is a variation in conditions (tolerance of the bushing diameter or of the hole diameter, or of the chemical composition of the plastic).
Another known type of the thread insert which does not require forcible installation, consists of a coil of wire of lozenge-shaped cross-section. This wire coil is screwed into a hole in the mounting base which has previously been tapped. It offers advantages through the elasticity of the wire coil which forms the screw thread but is obviously not used for the same purpose nor is it economically competitive with inserts whose principal purpose is to save the tapping operation.